Spatial technique for measuring modal delay differences in a dual-mode optical fiber

A simple spatial method using a coherent light source and an interferometer is proposed for measuring the group delay time difference between the LP(01) and LP(11) modes in a dual-mode optical fiber. Experimental results obtained by this method are in good agreement with those obtained by the swept-frequency and pulse methods. Moreover, a group delay time difference of 4 psec is measured. Results suggest that this spatial. method can be applied to a short fiber less than 1 m in length and/or small time differences around 1 psec by using a laser diode with a short coherence length.